Projection systems are classified into three-panel projection systems and single panel projection systems according to the number of light valves which form an image by controlling the on/off operation of light emitted from a high power lamp used as a light source on a pixel-by-pixel basis. The single panel projection system may have an optical system smaller than that of the three-panel projection system in size. However, since the single panel projection system splits white light sequentially into three light beams of red (R), green (G), and blue (B) colors, a light efficiency of the single panel projection system decreases to ⅓ of that of the three panel type projection system. Thus, attempts to increase the light efficiency of the single panel projection system have been made.
A conventional single panel projection system is disclosed in U.S. Application No. 2002/191154 A1. As shown in FIG. 1, in the disclosed conventional single panel projection system, white light emitted from a light source 100 passes through first and second lens arrays 102 and 104 and a polarized light beam splitter array 105, and is then split into R, G, and B color light beams by first, second, third, and fourth dichroic filters 109, 112, 122, and 139. R and G color light beams are transmitted through the first dichroic filter 109 and proceed along a first optical path I1, and a B color light beam is reflected by the first dichroic filter 109 and proceeds along a second optical path I2 to be reflected by a mirror 133. The R and G color light beams proceeding along the first optical path I1 are separated from each other by the second dichroic filter 112. In other words, the R color light beam is transmitted through the second dichroic filter 112 and proceeds along the first optical path I1 to be reflected by another mirror 138, and the G color light beam is reflected by the second dichroic filter 112 and proceeds along a third optical path I3.
As described above, the white light emitted from the light source 100 is split into the R, G, and B color light beams, and the R, G, and B color light beams are scrolled to pass through first, second, and third prisms 114, 135, and 142 respectively corresponding to the R, G, and B color light beams. The first, second, and third prisms 114, 135, and 142 are arranged on the first, second, and third optical paths I1, I2, and I3, respectively, and rotated at a uniform speed so that R, G, and B color bars are scrolled. The G and B color light beams proceeding along the second and third optical paths I2 and I3 are reflected by and transmitted through the third dichroic filter 139 so that the G and B color light beams are combined. Finally, the R, G, and B color light beams are combined by the fourth dichroic filter 122 and pass through the polarized light beam splitter 127, and a light valve 130 forms an image using the R, G, and B color light beams. Here, reference numeral 125 denotes a polarizer, and reference numerals 118 and 133 denote light path conversion units.
FIG. 2 illustrates a process of scrolling R, G, and B color bars due to rotations of the first, second, and third prisms 114, 135, and 142. Here, the R, G, and B color bars formed on a surface of the light valve 130 periodically move when the first, second, and third prisms 114, 135, and 142 corresponding to the R, G, and B color light beams are rotated at the same time and at the same speed. For example, if R, G, and B color bars are formed on the light valve 130, one frame of color image is produced when the R, G, and B color bars rotate one round as shown in FIG. 2.
The light valve 130 forms a color image by processing its individual pixels according to an on-off signal. A projection lens (not shown) magnifies and projects the color image onto a screen.
In the above-mentioned method, since an optical path has to be used for each of three color light beams, a lens for each of the three color light beams is required, and parts for condensing split light beams are necessary. Thus, a volume of the single panel projection system is increased, assembly thereof is difficult, and optical paths are complicated to cause a difficulty in arranging an optical axis. Also, etendue of the light beams is increased during a process of splitting light into the three color light beams and condensing the three color light beams. Here, the etendue E refers to an optical conservative physical quantity in an optical system and is expressed as in Equation 1.
                    E        =                              π            ⁢                                                  ⁢            A            ⁢                                                  ⁢                                          sin                2                            ⁡                              (                                  θ                                      1                    2                                                  )                                              =                                    π              ⁢                                                          ⁢              A                                                      (                                  4                  ⁢                  Fno                                )                            2                                                          (        1        )            wherein A denotes an area of an object, the etendue of which is to be measured, and Fno denotes F numbers of lenses. In Equation 1, the etendue depends on the area and F number of the object and refers to a physical quantity expressed by a geometrical structure of the optical system. The etendue at a starting point of the optical system should be identical to the etendue at an ending point of the optical system, so that the light efficiency is optimized. For example, if the etendue at the ending point of the optical system is greater than the etendue at the starting point, the volume of the optical system is increased. If the etendue at the ending point is less than the etendue at the starting point, light may be lost. If the etendue of the light source is great, angles of the light beams incident onto a subsequent lens are widened. Thus, it is difficult to constitute the optical system satisfying these requirements.
However, the optical system may be easily constituted while reducing the etendue.
However, in a single panel scrolling projection display device, a light beam is split into three color light beams and then condensed. Due to this, a divergence angle becomes bigger, and thus the etendue is increased. Thus, constituting an optical system becomes difficult due to an increase in the etendue.
Also, in a general single panel projection optical system, white light is split into R, G, and B color light beams, which are sequentially forwarded to a light valve by a filter. The light valve operates according to the order of R, G, and B color light beams to form a color image. As described above, since the single panel projection optical system sequentially uses the R, G, and B color light beams, the light efficiency decreases to ⅓ of that of the three panel optical system. In order to solve these problems, a color scrolling method was suggested. In the color scrolling method, after the white light is split into the R, G, and B color light beams, the R, G, and B color light beams are forwarded to different positions of the light valve at the same time. All of the R, G, and B color light beams have to reach one pixel to realize a color image.
Thus, each of the R, G, and B color light beams is moved at a uniform speed using a specific method.
In the conventional single panel projection optical system, when prisms are rotated for scrolling, an independent prism is used for each of color light beams. However, it is difficult to match respective driving velocity of the prisms and the light valves, and the velocity for scrolling the color light beams may not be uniform due to circular movements of the prisms. Since a separate part is necessary for each of the color light beams, the volume of the optical system is increased, and processes of manufacturing and assembling the optical system are complicated. As a result, yield of the optical system decreases.